System level specifications for power converters have become increasingly stringent in the last years. This is especially the case with regard to power converters used in portable electronic devices. On the one hand, power converter operating efficiency is critical as it has a direct influence on battery lifetime as well as power density and the related form factor of the portable electronic device. On the other hand, the power converter must not only provide operating power for the device, but must also provide a regulated output to manage battery charging functions.
Multi-level power converter topologies such as e.g. the multi-level buck converter are a promising approach to alleviate many of the shortcomings of conventional power converter topologies. In a multi-level buck converter, for example, two high-side switches connected in series replace the single high-side switch of the traditional buck converter. Moreover, two low-side switches connected in series replace the single low-side switch of the traditional buck converter and a so-called flying capacitor is connected in parallel to the series connection of the lower high-side switch and the higher low-side switch.
A major advantage of the multi-level buck converter is that the root mean square RMS voltage on the inductor node is only 50% of the corresponding voltage of a traditional buck converter. In addition to reducing the RMS voltage across and the RMS current though the inductor, the voltages across the switching capacitors is also reduced, thus lowering switching losses. Furthermore, transistors with lower breakdown voltage ratings typically have lower drain-source resistances Rds, resulting in reduced conductive losses.
However, there is a need to control the flying capacitor voltage. This is in particular true in no-load or light load operating conditions. For instance, during light load conditions, the power converter may be controlled such that the two high-side switches are switched simultaneously and the two low-side switches are switched simultaneously. In this exemplary scenario, ideally, the two high-side switches (or the two low-side switches, alternatively) should turn off at the same time and the voltage over the flying capacitor should float at e.g. half of the input voltage of the power converter. But non-idealities in the transitions cause one switch to turn off earlier than the other, causing the flying capacitor to either charge high to the input voltage or discharge to ground. The resulting charging and discharging of the flying capacitor may trigger fault conditions. Specifically, the charging of the flying capacitor may cause an over-voltage OV alarm, whereas the discharging of the flying capacitor may cause an under-voltage UV alarm. In response to the OV alarm, the flying capacitor voltage is lowered and, in response to the UV alarm, the flying capacitor voltage is increased. But the triggering of these alarms and the subsequent correction of the flying capacitor voltage wastes power and may also cause audible noise.